Internal laser drilling, sometimes abbreviated KM after the Hebrew kiduach meyuhad meaning "special drilling", is a clarity treatment in which a focused laser beam reaches a dark inclusion through pre-existing internal feathers or fractures rather than burning a fresh channel from the surface. The technique was developed in the 1990s by the Israeli diamond industry and is recognised by every major laboratory as a treatment that must be disclosed.

How conventional laser drilling works

Conventional laser drilling, in use since the early 1970s, fires a tightly focused infrared laser into a diamond and burns a hair-fine channel from the surface to a dark inclusion, typically a graphitic or sulphide crystal. The inclusion is then bleached with acid pumped through the channel, lightening or removing it visually. The straight surface-reaching channel is the diagnostic feature: under magnification it looks like a fine white tube running from a tiny pit on the surface to a now-pale inclusion in the interior.

What changes in internal drilling

Internal laser drilling exploits an existing internal feather that already reaches the inclusion or comes close to it. The laser is aimed so that it widens or extends the natural feather to connect with the inclusion, again allowing acid to bleach the dark crystal. Crucially, no new straight channel reaches the surface; the path follows the irregular geometry of the feather. Under the loupe the result is an enlarged, more transparent feather rather than the textbook drill tube, and the surface above shows no pit.

Identification

Although the surface entry point is absent, internal drilling leaves recognisable evidence. The treated feather often shows a slightly squared or stepped edge where the laser has eaten into the crystal beyond the original break. The bleached inclusion sits at the inner end of the feather, sometimes whitened to a chalky residue rather than removed entirely. Occasionally a series of small bubble-like cavities mark the laser path. Under polarised light the strain pattern can differ from that around an untreated feather. Major laboratories disclose internal drilling on grading reports.

Trade and disclosure

FTC guidelines in the United States and the equivalent rules in the European Union, Japan and most other markets require disclosure of laser drilling, including the internal variant, at every stage of sale. The treatment is permanent and stable under all normal handling and repair conditions, and it does not weaken the stone in a way that distinguishes it from an untreated diamond with similar feathers. However, an internally drilled stone is priced below an equivalent untreated stone, typically at a discount in the range of 10 to 25 per cent depending on the visibility of the treated feature.